Conventional solvent-based industrial finishes and coatings have presented many problems to date. Organic solvents can pollute the workplace and environment. In addition, many organic solvents are readily ignited, toxic, expensive, and lower the quality of finishes, and they can add undesirable colors to an otherwise colorless finish. As a replacement for these solvent-based finishes, the trend in the polymer industry has been toward high solids, liquid coatings and polymerization processes reducing or eliminating solvent.
High solids coatings offer significant advantages over conventional, solvent-thinned coatings. For example, high solids coatings do not pollute the air, they reduce or eliminate exudation of fumes in use, they reduce the energy requirements for their preparation in terms of material, energy expended and labor, and unlike solvent-based systems, they do not present significant fire and toxicity problems. High solids coatings also provide substantial advantages over other high solids liquids, such as solventless, water-borne, powder and non-aqueous systems. Additionally, they offer a better balance of properties.
One of the most enduring and difficult problems in preparing and utilizing high solids coatings is the selection and control of viscosity. Significant efforts have been made to improve processes and polymeric products for use in high solids applications. For example, various methods of characterizing polymers such as the number average molecular weight (Mn), weight average molecular weight (Mw), and the sedimentation average molecular weight (Mz) and ratios calculated using these characteristics such as the polydispersity ratio (Mw/Mn) and the dispersion index (Mz/Mn) have been investigated and adjusted to obtain polymeric products with viscosities useful for high solids applications. Although these efforts have produced vast improvements in polymers for use in high solids applications, improved processes are required to produce polymers with characteristics maximized for high solids applications.
U.S. Pat. No. 4,414,370 discloses a continuous bulk polymerization process for polymerizing vinylic monomers to prepare low molecular weight polymers employing thermal initiation at reaction temperatures from 235° C. to 310° C. and residence times of at least 2 minutes in a continuous stirred reactor zone. The vinylic monomers of the disclosed process include styrenic monomers such as styrene and α-methylstyrene; acrylic monomers such as acrylic acid, methacrylic acid, acrylates, methacrylates; and other non-acrylic ethylenic monomers such as vinyl acetate.
U.S. Pat. No. 4,529,787 discloses a continuous bulk polymerization process including an initiator for preparing low molecular weight, uniform polymers from vinylic monomers at short residence times and moderate reaction temperatures to provide high yields of a product suitable for high solids applications. The disclosed vinyl monomers include styrenic monomers such as styrene and a-methylstyrene; acrylic monomers such as acrylic acid, methacrylic acid, acrylates, methacrylates, and functional acrylic monomers; and non-acrylic ethylenic monomers such as maleic anhydride and vinyl pyrrolidone.
U.S. Pat. No. 4,546,160 discloses a continuous bulk polymerization process for polymerizing acrylic monomers to prepare low molecular weight, uniform, polymers for use in high solids applications which uses a minor amount of initiator at short residence times and moderate temperatures.
U.S. Pat. No. 5,130,369 discloses a batch process for preparing functionalized polymeric compositions. The functionalized polymeric compositions are prepared by polymerizing an ethylenically unsaturated functional monomer in a solvent including a reactive compound. The polymeric compositions prepared according to the process can be used as builders in detergent compositions, as pigments, dispersants in coating compositions, as tanning agents for leather, as associative thickeners and as rheology modifiers in coating compositions.
U.S. Pat. No. 5,521,267 discloses a batch process for preparing polymers from ethylenically unsaturated compounds containing acid groups with further ethylenically unsaturated compounds and monohydroxy compounds.
The polymer industry has long known that continuous polymerization processes are best for obtaining large quantities of polymeric product. Furthermore, optimized continuous processes provide economic advantages over batch polymerization processes and may provide more uniform polymeric products. While continuous processes have been disclosed for the preparation of certain polymeric products for use in high solids coatings applications, a continuous process is needed for preparing polymeric products with improved viscosity characteristics in aqueous media for use in high solids applications. Furthermore, a need remains for a polymerization process which incorporates an esterification reaction in which an alcohol with desirable viscosity-modifying behavior is incorporated into a polymeric chain with a high degree of conversion.